Communication via radio frequency (“RF”) devices is regulated by national and international regulatory agencies in order to ensure maximum utilization of limited spectral resources and to minimize interference. In the United States of America, the Federal Communication Commission (“FCC”) regulates and licenses specific portions of radio frequency spectrum or bands for broadcast and other forms of RF communication.
A number of bands have been set aside for “Industrial Scientific and Medical” use, or the (“ISM”) bands by the FCC. Utilization of these bands are unlicensed but is regulated by the FCC. For example, the 900 MHz band is used by a number of consumer wireless devices, such as cordless phones. The Wireless LAN under the IEEE 802.11 standard can have its physical layer operate in 2.4 GHz. Another unlicensed band is at 5.9 GHz.
The FCC regulation governing these ISM bands are documented in “Operation with the bands 902–928 MHz, 2400–2483.5 MHz and 5725–5875 MHz”, Title 47 Part 15 Section 247) Code of Federal Regulations (47 CFR 15.247). The regulation stipulates the operation of either a frequency hopping or direct sequence spread spectrum intentional radiators. The regulation is based on consideration of reusing the same bands in multiple locations. Hence, transmissions in these bands are necessarily limited to low power applications, yet with high rejection of noise. When implementing with spread spectrum schemes the regulation specifies specific power spectrum density that must be adhered to.
FIG. 1 illustrates a frequency hopping spread spectrum scheme in which a transmission band is partitioned into multiple channels. The transmission spectrum is delimited by a lowest frequency f1 and a highest frequency f2. If the transmission spectrum is partitioned into N channels, each channel will have a bandwidth of ΔfC=(f2-f1)/N.
FIG. 2A shows a conventional frequency hopping spread spectrum communication system having a transmitter and a receiver. The transmitter transmits data through a pseudo-random sequence of channels in time. A local oscillator in the form of a voltage-controlled oscillator generates the carrier frequency band of a given channel. The VCO puts out a given channel at a time according to a sequence of predetermined pseudo-random numbers. In this way, the transmitter transmits equally on average over all the different channels in the transmission spectrum. The data is then said to be spread over the transmission spectrum even though at any one time it is confined to one of the channels.
The receiver includes a synchronization circuit that essentially helps tune into the current transmitting channel and the tuning must hop in synchronization with the transmission.
FIG. 2B illustrates the frequency hopping synchronization of channels between the transmitter and the receiver of FIG. 2A. Various schemes exist for synchronization. For example, a protocol provides a one-way hand-shaking to allow the receiver to lock into steps with the same pseudo-random sequence of the hopping transmitter. Another scheme allows the overheads in the transmission to tell the receiver where the next channel will be.
The conventional frequency hopping system has the transmission and reception synchronously spread over different narrow band channels at different times. This provides a communication system with high signal to noise ratio and low interference.
However, the need for synchronization of the receiver to the transmitter in a conventional frequency hopping system increases complexity and the need for higher precision in both the receiver and the transmitter. This results in more complicated and more expensive equipment and operation.